1. Field of the Invention
This invention relates to the field of vestibular testing, more particularly to a method and apparatus for the clinical evaluation of the balance organs of the inner ear.
2. Description of Related Art
Dizziness is one of the most common clinical complaints around the world. Functional testing of the vestibular system (the balance organs of the inner ear) is often necessary for correct diagnosis and treatment, but accurate tests are costly and complex.
It has been reported that dizziness is the most common motive for consultation of a physician in patients over seventy five years of age. While in some cases the symptoms are typical and a cause is easily established, in many other cases a specific diagnosis depends on precise functional testing, particularly of the vestibular system. At the present time, this testing is very costly, and requires highly trained operators to interpret the results.
The inner ear consists of two parts, the cochlea that transduces sound waves and allows us to hear, and the vestibular labyrinth that senses movement and generates reflexes to stabilize our eyes, heads and bodies. Usually, acute damage to the labyrinth will produce false sensations of movement, often called vertigo or dizziness, as well as gaze and postural instability and motion sickness. These symptoms tend to become increasingly vague, however, as compensation develops. As a result, it can be difficult to distinguish between vestibular and other problems of a quite different nature. Of course, prognosis and treatment depend heavily on a correct diagnosis.
One common method of testing the vestibular system consists of squirting warm or cool water into each ear canal and monitoring the resulting xe2x80x9ccaloric nystagmusxe2x80x9d. These involuntary eye movements result largely but not exclusively from thermally-induced convection currents in endolymph, particularly in the horizontal canal. The results of caloric testing are combined with separate assessments of the eye movement control system and of eye movements that result from changes in the patient""s position. This is usually referred to as the electronystagmogram (ENG) test battery. While helpful in some cases, it often provides equivocal data in patients with mild symptoms, and these are precisely the ones for which an objective test is necessary.
The only way to apply a selective and controlled stimulus to the vestibular system is to expose the patient to physical movement. Furthermore, this movement must be of a sort that makes it impossible for predictive or other compensatory mechanisms to hide deficits in vestibular function. Normally, this means fast, because the neural circuits underlying compensation are more complex and hence slower than the very simple pathways used by vestibular reflexes. However, sudden movements require powerful machines, such as high-torque servo-controlled rotators, as in rotary chair apparatus. A safe, man-rated version of these devices can be costly, require recurrent inspections, maintenance and repairs. As a result, such devices and the sophisticated vestibular testing they make possible have been limited to a relatively few university teaching hospitals.
Australian researchers Ian Curthoys and Michael Halmagyi (Halmagyi, G. M., Curthoys, I. S., Cremer, P. D., Henderson, C. J., Todd, M. J., Staples, M. J. and D""Cruz, D. M. The human horizontal vestibulo-ocular reflex in response to high-acceleration stimulation before and after unilateral vestibular neurectomy. Exp. Brain Res. 81: 479-490, 1990.) have demonstrated a far simpler technique that measures eye movements during the first 100 msec of an unpredictable, passive horizontal head rotation (peak head displacement 20xc2x0, peak head velocity 200-300xc2x0/sec, peak head acceleration 2000-4000xc2x0/sec2). When eye velocity is plotted as a function of head velocity in patients with an unilateral loss of vestibular function, eye responses during rotations toward the intact side are found to be close to normal but the responses are found to be markedly decreased when the rotation is toward the lesioned side. Most significantly, this deficit appears to be permanent, implying that it cannot be hidden by compensatory mechanisms.
This work is a significant step towards a simple and practical clinical test of human vestibular function, but it has one serious drawback. To reach significant head angular velocities in less than 100 msec, large head angular accelerations are necessary. These are achieved by the examiner abruptly and unpredictably rotating the patient""s head only. This movement would inevitably be opposed by involuntary stretch reflexes and could lead to severe consequences in the presence of undetected cervical spinal pathology. While similar, rapid head rotations can be produced voluntarily, such a method would suffer badly from predictability of the stimulus. As well, not everyone can perform the maneuver in an acceptable fashion.
There is a need to provide a simple, effective and inexpensive technology that would allow diagnostic testing in many additional hospitals, clinics and offices.
According to the present invention there is provided a method of evaluating the vestibular function in a human subject, wherein the human subject is constrained in a substantially erect position so that the head moves in unison with the rest of the body, a controlled stimulus in the form of a sudden angular acceleration is imparted to the body, and the subject""s ocular response to the controlled stimulus is measured.
Typically, the subject is placed in a pivotable mechanism, such as a frame that can pivot essentially about the vertical axis or another axis. The subject is essentially erect within the frame and is stabilized in a vertical standing position, or in a horizontal position, or in an inclined position relative to the vertical. The subject""s head is substantially immobile relative to the torso or the rest of the body. The head may be immobilized by various means, such as a clamp consisting of a pair of brackets.
After the initial acceleration the subject is preferably brought to a rapid standstill with the aid of a fluid damping mechanism that exploits the fact that during an initial period of laminar flow, the resistance to motion of a body moving in a fluid is very low, and then with the onset of turbulent flow, this resistance suddenly increases.
The subject""s response to this controlled stimulus is preferably measured using electro-oculography (EOG), which is the recording of electrical signals produced by eye movement. The stimulus is recorded using angular velocity transducers. Surface electrodes used in conjunction with a forehead-mounted ring and adjustable connectors are a convenient way of performing electro-oculographic recording. Angular velocity transducers are preferably secured in place with a head adaptor allowing for the precise measurement of the angular speed of the head without slip.
Vestibular function can be assessed using eye versus head velocity data. Eye velocity is derived from eye position data. Further decision analysis can be based on eye velocity versus head velocity curves.
An alternative approach would be to use a man-powered rotator, including a seat, a footrest and a means of securing the subject""s upper body and head to the chair. Unfortunately, such a device has great difficulty achieving adequate angular acceleration. Fully half of one""s body mass is located in the legs. In the sitting position, this is distributed far from the axis of rotation, greatly increasing angular inertia. Such a human powered head rotator type could not reach the angular accelerations necessary for the method of Curthoys and Halmagyi. The applicants have found surprisingly that the method of the invention achieves the necessary angular rotation in a simple and effective manner.
In another aspect the invention provides an apparatus for evaluating vestibular function in a human subject comprising a frame for supporting the human subject in a substantially erect position with the head constrained so that it moves in unison with the rest of the body, the frame being rotatable about a longitudinal axis of the subject, an arrangement for measuring ocular response to a controlled stimulus in the form of a sudden angular acceleration imparted to the frame, at least one sensor for measuring the angular velocity of the subject""s head, and a processing unit for deriving the vestibular function from the subject""s ocular response and the angular velocity.